Half of the sun's power reaching the earth lies in the infrared, but this power is currently underutilized in large-area, low-cost photovoltaics. Early progress towards infrared solution-processed photovoltaics has emerged in organic materials; however, low-bandgap conjugated polymer/fullerene-derivative bulk heterojunctions remain sensitive only to 1000 nanometers (nm). Several other efforts have focused on sensitizing organic devices to the infrared using conjugated polymers/nanocrystal composites, but efficiencies are still well below the best organic bulk-heterojunction devices. Recent results have shown that it is possible to make efficient photovoltaic devices comprising only nanocrystals films; the most efficient nanorod heterojunction devices show response to 800 nm.
Conjugated polymers have been widely investigated and have shown promising efficiencies. However, they remain transparent in most of the infrared spectral region. Because half the sun's energy lies in the infrared, the optimal bandgap for a single-junction solar cell lies in the infrared, well beyond the sensitivity of today's organic solar cells.
In contrast with organics and polymers, colloidal quantum dots (CQDs) offer tuning to access different spectral regions through simple variation of their chemical synthesis. By virtue of their size-tunable optical properties, lead salt colloidal quantum dots (CQD) can be engineered to access the visible and the short-wavelength infrared spectral regions. Recently, organic polymers sensitized using infrared lead salt nanocrystals have been investigated; however, these devices did not exceed monochromatic power conversion efficiencies of 0.1%. Relatively higher (e.g., 1.3%) monochromatic infrared power conversion efficiencies have been reported through the use of thiols and high temperature processes to achieve smooth films on rough nanoporous transparent metal oxides. The highest infrared monochromatic external quantum efficiencies (EQE) achieved has been reported as 37% under 12 mW cm−2 illumination at 975 nm. These PbS CQD-based devices registered an infrared power conversion efficiency of 4.2%.
Solution-processed photovoltaics offer solar energy harvesting characterized by low cost, ease of processing, physical flexibility, and large area coverage. Conjugated polymers, inorganic nanocrystals (NCs), and hybrid materials have been widely investigated and optimized to this purpose. Organic solar cells have already achieved 6.5% solar conversion efficiencies. However, these devices fail to harvest most of the infrared (IR) spectral region. High efficiency multijunction solar cells offer the prospect of exceeding 40% efficiency through the inclusion of infrared-bandgap materials. In this context, infrared single junction solar cells should be optimized for infrared power conversion efficiency rather than solar power conversion efficiency. For double and triple junction solar cells, the smallest-bandgap junction optimally lies at 1320 nm and 1750 nm respectively. Attempts to extend organic solar cell efficiency into the near infrared have so far pushed the absorption onset only to 1000 nm.